The most widespread structure for a torsion box is formed using front and rear spars and a plurality of ribs transversally arranged and connected to the front and rear spars, to form a box-like configuration. The torsion box also includes upper and lower skin panels internally reinforced by stringers. The main functions of the ribs are: to provide torsional rigidity, to longitudinally limit the skins and the stringers so as to discretize buckling loads, to maintain the shape of the aerodynamic surface and to support local load introductions resulting from actuator fittings, support bearings, and similar devices, which are directly secured to the front and rear spars.
Another known structural concept for a torsion box is the “multi-spar” concept, wherein the ribs are dispensed with and several spars are introduced for creating closed cells in order to replace the functions of the ribs of the previous concept. The multi-spar concept is typically used for the construction of the HTP of an aircraft, also known as a horizontal stabilizer.
FIGS. 1A and 1B show an example of a prior art multi-spar torsion box for a HTP created from a Carbon Fiber Reinforced Plastics (CFRP) material. The HTP (1) is formed by two lateral torsion boxes (1′,1″) (right-hand side and left-hand side torsion boxes) joined to each other at the central region of the HTP (1), and symmetrically arranged with respect to the axis of symmetry of the horizontal tailplane.
The torsion box is divided in three cells (3) by means of eight C-shaped spars grouped in pairs. The intermediate C-shaped spars (4) have, extended spar flanges and are folded at the free edges to form a stringer web (9) together with the adjacent spar. The multi-spar structure of the example also integrates the rear and front spars (7,8), and upper and lower covers (skins) (5,6). Other components of the HTP structure, such as leading and trailing edges and elevators, are assembled within the torsion box.
The European Patent Application No. EP2153979A1, describes a multi-spar torsion box with more detail.
A multi-spar torsion box has the advantage that its construction is simple, however, it is known that multi-spar torsion boxes have problems transmitting highly concentrated transverse loads, hence they may be affected by deformations caused by transverse loads. The reason for this drawback is that in a multi-spar torsion box there are no transversal ribs. Therefore, since the multi-spar concept as such does not have much torsional rigidity, sometimes, it is necessary to reinforce the structure by using additional reinforcement elements such as ribs, bars or other type of reinforcement elements, fitted at critical areas of the structure.
This is the solution proposed in the U.S. Patent Publication No. 2009/0001218A1, wherein several brackets and diagonal bars are fitted into the structure, in order to reinforce the same against torsional deformations.
However, the installation of these reinforcement elements inside the torsion box is cumbersome and time-consuming, because a multi-spar torsion box is a closed structure, hence the access to the interior is difficult. Furthermore, the installation of these reinforcement elements, increase the manufacturing and assembly costs and increases as well the weight of the structure.